The present invention relates to electrolytic capacitors of the kind including a formed valve-metal foil anode spaced from a cathode and a liquid electrolyte therebetween, and more particularly relates to a controlled method for etching the valve metal foil anode prior to forming to increase the actual valve-metal surface area and thus increase the capacity of the capacitor.
It is well known that etching anode and cathode valve-metal foils increases the actual surface area to be "formed". The "forming" process results in a valve-metal oxide film being formed over all exposed surface portions of the foil including the etch pits, cavities, pores or whatever irregularities that were caused by the etching. Since the capacitance is directly proportional to the area of the anode surface, it is desirable to achieve at etching a high ratio of actual area to apparent area. The term "apparent area" as used herein is the planar area of the etched foil that is equal to the area existing before etching. Such ratios are referred to as etch ratios or gain and may range in value from 10 to 50. For a particular foil having an etch gain of 10, equal sized test squares of the unetched and the etched material both formed by the same process will exhibit capacitances, respectively, in the ratio of 1:10. Lower etch ratios are generally preferred for heavily formed high voltage anode foil, while high gain foil is used in capacitors having low voltage ratings.
It is also desired to control the etching process so as to produce etched foil having a constant predetermined ratio of actual to apparent areas. Toward this end, in a continuous etch process, it is customary to periodically cut out a sample of the freshly etched foil, form it to a predetermined voltage and measure the characteristic capacity of the formed sample in an electrolytic cell. This means of controlling the etch process is not only slow but is also complex, subject to error and is costly in material and in terms of manpower requirements. Such etched foil normally varies about .+-.10% from a nominal gain, and at least 10% more foil is employed in each capacitor than would be necessary if the foil had an invarient gain.
The great variety of etch processes that have been used for one special type capacitor or another include DC and AC electrochemical methods as well as pure chemical etch methods. Each of those etch methods tends to produce its own characteristic material removal pattern. Foil thickness as determined by use of a mechanical micrometer, may not change at all during most etch processes. For some etch processes, the amount of material removal, as determined by weight loss measurements, tends to be a rather linear function of the ratio of actual to apparent surface areas. But for others, this relationship is quite non-linear. In any case, the use of weight loss measurements to control etched surface area is like the capacity measurement itself not suitable for use as a fast response continuous measure of etched foil surface area.
It is an object of the present invention to provide a method for continuously monitoring the etch ratio and altering at least one etch process parameter to maintain a nearly constant etch ratio.
It is a further object of the present invention to provide substantially etch-ratio-invarient-foil to permit the manufacture of less costly and smaller electrolytic foil capacitors.